The present invention relates generally to testing machines for fiber-matrix composite materials, and more particularly to loading probes for fiber-matrix testing machines.
Fiber push-in and push-out has been used to measure interface properties in ceramic and metal matrix composites since the early 1980's. Interface properties are important for a variety of reasons. For example, as part of means for preventing crack propagation, some sliding may deliberately be allowed to occur at the interface between the fibers and the matrix in ceramic and metal matrix composites. Testing apparatus is required to measure the adhesion between the fibers and the matrix surrounding them to determine the effectiveness of various coatings and treatments for varying interface properties. Push-out type apparatus use a loading probe or probes to push on single or multiple fibers imbedded in a sectioned fiber-matrix composite. Variously placed sensors measure the displacements and forces on the probes and fiber-matrix test sample. A number of analytical approaches have been proposed in the past for analyzing the sensor data to derive interface properties, one of the most rigorous of which to date is the model of R. J. Kerans and T. A. Parthasarathy described in their paper, "Theoretical Analysis of the Fiber Pull-out and Push-out Tests," J. Am. Ceram. Soc., 74 [7], p 1585-1596 (1991), which is incorporated by reference.
Because there are no commercial push-out apparatus available, each unit is unique and customized for the particular type of composite being examined. Tested fibers run from relatively large monofilaments (&gt;100 microns diameter) which are used in metal matrix and some ceramic matrix composites, to smaller textile (&lt;25 microns) diameter fibers which are preferred for ceramic matrix composites. Textile fibers are fibers small and flexible enough to be woven into cloth. Large monofilaments are relatively stiff and generally cannot be woven.
The first probes used as push-out apparatus probes were standard diamond pyramid indenters, such as the well-known Vickers indenter commonly used in microhardness testers. They were used because they were available. The Vickers indenter is a four sided pyramid. Also occasionally used was the related Burkovick indenter which is a three sided pyramid. Unfortunately, because those indenters are sharply pointed, they plastically deform the ends of fibers. They also commonly crack and chip the pushed fiber. Further, the geometry of those indenters results in a substantial portion of the applied load acting perpendicular to the axis of the fiber. Finally, the shape of those indenters severely limited the maximum allowable fiber displacement that could be achieved. These problems made it virtually impossible to derive reliable quantitative information from a push-out test.
After four or five years, the limitations of the early indenters were realized and researchers began to look for alternatives. Eventually, flat-ended cylindrical tungsten carbide probes came into common use. These probes are a standard commercial item for other uses and are commonly available, including in the small sizes necessary to push textile diameter fibers. Unfortunately, cylindrical tungsten carbine probes are not very strong, particularly at the very small diameters needed to push textile fibers. Even at the larger sizes for monofilament fibers, cylindrical tungsten carbide probes are not strong enough to perform push-out tests at the high loads required to test some of the metal matrix composites. The loads required to push-out large monofilament fibers from a metal matrix are so high that prior art fiber-matrix testing machines have had to use very thin test sections to sufficiently reduce the loads so that the prior art probes would not break. With such thinly sectioned test samples, the results of those push-out tests are very unreliable.
It is seen, therefore, that there is a need for probes for fiber push-out testers that are sufficiently strong at small diameters to perform push-out tests for small diameter textile fibers and sufficiently strong at larger diameters to push-out large monofilament fibers from sectioned metal matrix composites sufficiently thick so that reliable test results can be obtained, all without damaging the fibers or otherwise introducing errors into the tests.
It is, therefore, a principal object of the present invention to provide a new and stronger probe for fiber push-out testers that is strong enough to perform reliable push-in and push-out tests not possible with prior art probes.
It is a feature of the present invention that it is simple and straightforward to use.
It is an advantage of the present invention that it produces more reliable test results by minimizing damage to the fibers being pushed.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.